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Message-ID: <Zemy0bbZh7YA1aRY@pc636>
Date: Thu, 7 Mar 2024 13:28:01 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>,
"Paul E . McKenney" <paulmck@...nel.org>, RCU <rcu@...r.kernel.org>,
Neeraj upadhyay <Neeraj.Upadhyay@....com>,
Boqun Feng <boqun.feng@...il.com>, Hillf Danton <hdanton@...a.com>,
LKML <linux-kernel@...r.kernel.org>,
Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH 1/2] rcu: Do not release a wait-head from a GP kthread
On Thu, Mar 07, 2024 at 01:21:50AM -0500, Joel Fernandes wrote:
>
>
> On 3/6/2024 5:31 PM, Joel Fernandes wrote:
> >
> >
> > On 3/5/2024 2:57 PM, Uladzislau Rezki (Sony) wrote:
> >> Fix a below race by not releasing a wait-head from the
> >> GP-kthread as it can lead for reusing it whereas a worker
> >> can still access it thus execute newly added callbacks too
> >> early.
> >>
> >> CPU 0 CPU 1
> >> ----- -----
> >>
> >> // wait_tail == HEAD1
> >> rcu_sr_normal_gp_cleanup() {
> >> // has passed SR_MAX_USERS_WAKE_FROM_GP
> >> wait_tail->next = next;
> >> // done_tail = HEAD1
> >> smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> >> queue_work() {
> >> test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
> >> __queue_work()
> >> }
> >> }
> >>
> >> set_work_pool_and_clear_pending()
> >> rcu_sr_normal_gp_cleanup_work() {
> >> // new GP, wait_tail == HEAD2
> >> rcu_sr_normal_gp_cleanup() {
> >> // executes all completion, but stop at HEAD1
> >> wait_tail->next = HEAD1;
> >> // done_tail = HEAD2
> >> smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> >> queue_work() {
> >> test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
> >> __queue_work()
> >> }
> >> }
> >> // done = HEAD2
> >> done = smp_load_acquire(&rcu_state.srs_done_tail);
> >> // head = HEAD1
> >> head = done->next;
> >> done->next = NULL;
> >> llist_for_each_safe() {
> >> // completes all callbacks, release HEAD1
> >> }
> >> }
> >> // Process second queue
> >> set_work_pool_and_clear_pending()
> >> rcu_sr_normal_gp_cleanup_work() {
> >> // done = HEAD2
> >> done = smp_load_acquire(&rcu_state.srs_done_tail);
> >>
> >> // new GP, wait_tail == HEAD3
> >> rcu_sr_normal_gp_cleanup() {
> >> // Finds HEAD2 with ->next == NULL at the end
> >> rcu_sr_put_wait_head(HEAD2)
> >> ...
> >>
> >> // A few more GPs later
> >> rcu_sr_normal_gp_init() {
> >> HEAD2 = rcu_sr_get_wait_head();
> >> llist_add(HEAD2, &rcu_state.srs_next);
> >> // head == rcu_state.srs_next
> >> head = done->next;
> >> done->next = NULL;
> >> llist_for_each_safe() {
> >> // EXECUTE CALLBACKS TOO EARLY!!!
> >> }
> >> }
> >>
> >> Reported-by: Frederic Weisbecker <frederic@...nel.org>
> >> Fixes: 05a10b921000 ("rcu: Support direct wake-up of synchronize_rcu() users")
> >> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> >> ---
> >> kernel/rcu/tree.c | 22 ++++++++--------------
> >> 1 file changed, 8 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >> index 31f3a61f9c38..475647620b12 100644
> >> --- a/kernel/rcu/tree.c
> >> +++ b/kernel/rcu/tree.c
> >> @@ -1656,21 +1656,11 @@ static void rcu_sr_normal_gp_cleanup(void)
> >> WARN_ON_ONCE(!rcu_sr_is_wait_head(wait_tail));
> >>
> >> /*
> >> - * Process (a) and (d) cases. See an illustration. Apart of
> >> - * that it handles the scenario when all clients are done,
> >> - * wait-head is released if last. The worker is not kicked.
> >> + * Process (a) and (d) cases. See an illustration.
> >> */
> >> llist_for_each_safe(rcu, next, wait_tail->next) {
> >> - if (rcu_sr_is_wait_head(rcu)) {
> >> - if (!rcu->next) {
> >> - rcu_sr_put_wait_head(rcu);
> >> - wait_tail->next = NULL;
> >> - } else {
> >> - wait_tail->next = rcu;
> >> - }
> >> -
> >> + if (rcu_sr_is_wait_head(rcu))
> >> break;
> >> - }
> >>
> >> rcu_sr_normal_complete(rcu);
> >> // It can be last, update a next on this step.
> >> @@ -1684,8 +1674,12 @@ static void rcu_sr_normal_gp_cleanup(void)
> >> smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> >> ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
> >>
> >> - if (wait_tail->next)
> >> - queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
> >> + /*
> >> + * We schedule a work in order to perform a final processing
> >> + * of outstanding users(if still left) and releasing wait-heads
> >> + * added by rcu_sr_normal_gp_init() call.
> >> + */
> >> + queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
> >> }
>
> One question, why do you need to queue_work() if wait_tail->next == NULL?
>
> AFAICS, at this stage if wait_tail->next == NULL, you are in CASE f. so the last
> remaining HEAD stays? (And llist_for_each_safe() in
> rcu_sr_normal_gp_cleanup_work becomes a NOOP).
>
> Could be something like this, but maybe I missed something:
>
> @@ -1672,7 +1674,7 @@ static void rcu_sr_normal_gp_cleanup_work(struct
> work_struct *work)
> */
> static void rcu_sr_normal_gp_cleanup(void)
> {
> - struct llist_node *wait_tail, *next, *rcu;
> + struct llist_node *wait_tail, *next = NULL, *rcu = NULL;
> int done = 0;
>
> wait_tail = rcu_state.srs_wait_tail;
> @@ -1707,7 +1709,8 @@ static void rcu_sr_normal_gp_cleanup(void)
> * of outstanding users(if still left) and releasing wait-heads
> * added by rcu_sr_normal_gp_init() call.
> */
> - queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
> + if (rcu)
> + queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
> }
>
> /*
>
Unneeded queueing happens only first time after boot. After one cycle
we always need to queue the work to do a previous cleanups.
if (wait_head->next)
queue_the_work();
wait_head->next is always not NULL except first cycle after boot.
--
Uladzislau Rezki
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